Electro-acoustic apparatus



June 4, 1957 c. TQPHOLM 2,794,852

ELECTRO-ACOUSTIC APPARATUS Filed June 29, 1953 4 Sheets-Sheet l 2 may V x 19 H I b 6 8 if a 21 8 .UV IEWTOR; cfirv'sfl'an BY June 4, 1957 I Filed June 29, 1953 C. TGPHOLM ELECTRO-ACOUSTIC APPARATUS 4 Sheeizs-Shget 2 DVVZWYUR:

(Zr/152 i BY June 4, 1957 c. -rzPHoLM 2,794,852

ELECTRO-ACOUSTIC APPARATUS Filed June 29, 1953 4 Shets-Sheet 4 INVZZWJUR: CAP/Silk! 7;. Jaclyn/m,

United States Patent ELEcTRo-Acous'rrc APPARATUS Christian Topholm, Naerum, Denmark, assignor to American Danish Oticon Corporation, a corporation of Denmark Application June 29, 1953, Serial No. 364,878

Claims priority, application Denmark July 3, 1952 14 Claims. (Cl. 179-114) The present invention relates to electroacoustic devices.

More specifically the invention relates to electroacoustic devices of the electrodynamic type.

Such devices are a. 0. used in connection with hearing aid apparatus for instance in ear phones in which an alternating current having a frequency within the audible range is converted into sound waves to be transmitted to the ear of the user.

Such apparatus are of very small dimensions and considerable accuracy is therefore required in connection with their production and mounting.

One object of my invention is to provide a device of the kind here referred to which is constructed in such a manner that the mounting of the parts is facilitated so as to require less skilled labour and less number of operations.

Another object of my invention is to design a device of the kind specified in such a manner that a magnetic system and a diaphragm can be arranged in mutual spaced relationship with an air gap of a predetermined size.

Another object of my invention is to provide an electrodynamic device in which a magnetic system and a diaphragm adapted to be held in position spaced from the magnetic system with a predetermined air gap can be mounted on the same supporting member in a manner which enables the use of rational mass production.

Still another object of my invention is to provide an electrodynamic device in which a simplified adjustment of the air gap between a membrane and a magnetic system can be achieved.

Still another object of the invention resides in providing a supporting member for a magnetic system and a diaphragm in an electrodynamic device which can be manufactured by means of cutting tools only, and in which the air gap between the magnetic system and the diaphragm can be adjusted safely with a stamping operation.

Another object of the invention resides in providing an electroacoustic device for use as a hearing aid ear phone which can easily be adjusted to a sensibility within a predetermined frequency range.

Still a further object of the invention resides in providing an electroacoustic structure which is as well applicable as a sound reproducer such as in an ear phone as an electrodynamic microphone.

With these and still further objects in mind, which will appear from the following specification, the invention will now be described in more detail with reference to the drawing, in which Fig. 1 is a general perspective view of the top part of a hearing aid amplifier connected with an ear phone,

Fig. 2 is a perspective view of the various parts of an electroacoustic device according to the invention in the embodiment of a hearing aid ear phone with the parts separated axially,

Fig. 3 is an axial section through an ear phone assembled from the parts as shown in Fig. 2,

Fig. 4 is an axial section through the magnetic system of the ear phone as shown in Fig. 3 in enlarged scale,

ice

Fig. 5 is a plane view of the magnet supporting member used in the ear phone as shown in Figs. 2 and 3,

Fig. 6 is a perspective view of the half part of the magnet supporting member as shown in Fig. 5,

Fig. 7 is a section through a modified embodiment of the device as shown in Fig. 3, in which the device is de signed as an electrodynamic microphone.

Fig. 8 is a section through the magnet and membrane supporting member of the device shown in Fig. 7, and

Fig. 9 is a plane view of the sound receiving membrane of the device shown in Fig. 7.

In Fig. 1 there is shown a general view of the parts of a hearing aid apparatus comprising a casing adapted to fit in the pocket of the user and preferably to be secured in position of the pocket by means of a clip b so as to extend above the edge of the pocket with the top portion, in which is provided a plurality of conveniently patterned apertures c, behind which there is arranged a microphone adapted to receive sound waves to be transmitted to the ear of the user of the apparatus.

The housing a further comprises a convenient amplifier, which does not form part of the invention. Such amplifier is known to those skilled in the art and will also for this reason not be described in any detail.

On the top of the casing there is provided a switch operating member d for a switch located in the interior of the casing in order to switch on and off the amplifier. Furthermore there is located on the top of the casing a rotatable disc or dial e extending with a part of its periphery above the top surface of the casing a and operating a volume control of the amplifier so as to regulate the output thereof to an ear phone.

Finally there is provided outlet terminals (not shown) from the amplifier adapted to be connected with a plug 1, from which a flexible lead g is leading to another plug h plugged in terminal holes of an ear phone generally referred to by j, in which the alternating current responding to the sound waves received through the amplifier microphone is converted into acoustic Waves transmitted through an acoustic outlet k. This outlet is connected with an ear piece I moulded to fit the ear of the user either directly or through an intermediate pipe m.

Reverting now to the specific device which forms part of the invention the ear phone i is shown in more detail in the sections of Figs. 3 through 6 and with the parts separated axially in Fig. 2.

In general the magnet system comprises at least one pair of permanent magnets so as to produce a close magnetic field which is varied in response to variations of an air gap between a membrane and the poles of the magnetic system.

Such air gap variations may be effected electrically by applying an alternating current to the magnetic coil winding. When the frequency of such alternating current is within the audible range, a device of the character mentioned may be used as a sound reproducer.

It is also possible to vary the air gap between the membrane and the poles of the magnetic system by subjecting the membrane to mechanical vibrations caused f. inst. by a varying sound pressure. In such event an alternating current responding to the mechanical oscillations applied to the membrane will be induced into the coil windconnected by means of a magnetic yoke 2.

The space between the central magnetic core 1 and the exterior magnetic member 2 is occupied by a coil 3.

The annular magnetic member is supported on a member generally referred to by 5.

In devices of this kind a considerable accuracy of the air gap between the membrane and the magnet poles is required. The dimensions of devices for this hearing aid purpose are very small. The air gap may vary in accordance with the construction, but as a suitable size of air gap 17:100 millimetres will in apparatus of the specific type to be described hereinafter be of the magnitude applied. In order to obtain a satisfactory apparatus the allowed tolerance with respect to the air gap will be of the magnitude 1:100 millimetres.

This is obtained according to the invention by features to be described in the following.

This member is made of deformable material and is further provided with a portion 9 radially spaced from the annular magnetic member 4. Said portion 9 is adjusted axially relatively to the magnetic system soas to be able to support a membrane with a predetermined air gap in spaced relationship with respect to the magnetic system.

By performing the portion of the magnet supporting member adapted to support the membrane as a deformed portion which again is made possible by making the magnet supporting member of a deformable material, a simplification in the production is enabled inasfar as it is possible after having established the connection of the magnetic system with the magnet supporting member to measure the air gap between the poles of the magnetic system and a plane through the top of the deformed portion adapted to support the membrane and thereafter by a further deformation if necessary to adjust the size of the deformed portion in accordance with the desired air gap.

As shown specifically in the sections of Figs. 3 and 4, the supporting member 5 is overlapping the lower annular surface of the annular pole member 4.

This has the advantage that by using a material for the magnet supporting member having a large magnetic saturation induction, the magnetic flux will be concentrated in the overlapping portion of the magnet supporting member so as to thereby concentrate the magnetic field in a predetermined portion of the air gap, which again will increase the sensibility of the device.

In the actual embodiment disclosed the magnet 'supporting member 5 is in form of a washer of a deformable material.

Preferably the material of said washer is iron or soft steel without any substantial permeability, While as material for the annular magnet as well as the central core member 1 a nickel-iron alloy is used having a considerable magnetic remanence so as to be capable of forming a permanent magnetic system.

The magnet supporting washer 5 is further provided with a central aperture having an interior diameter substantially corresponding to the interior diameter of the exterior annular permanent magnetic member 4.

The washer and the annular permanent magnet are mutually connected in concentric relationship. In order to secure such mutual location the washer 5 is provided adjacent its center aperture with a depressed portion 5a of a radial extension corresponding substantially to the 'end surface of the annular magnetic member 4. Preferably the annular magnetic member is connected with the washer by means of soldering, but it will be understood that any other convenient form for connection can be applied such as f. inst. welding.

The central magnetic core 1 may be projecting slightly with its end surface 8 through the aperture of the magnet supporting washer. The annular deformed portion 9 of the washer is in accordance with a feature of the invention provided by means of deformation and forms a sup porting surface for the membrane 10.

This membrane is in accordance with another feature of the invention in form of a metallic diaphragm which is provided with an armature member 11.

Preferably this armature member has an exterior diameter corresponding to the interior diameter of the annular magnet 4, viz. the interior diameter of the center aperture of the armature supporting washer.

As it will be apparent from the above specification the annular magnetic member 4 is forming a closed wall within which the coil winding 3 is arranged.

As apparent from Fig. 2 there is provided in the magnetic yoke 2 a radial slot 2a opposite a hole 3a in the coil supporting member 3b. Outlet wires from the coil winding are projecting through the hole 3a and by means of the slot 2a the coil wires are furthermore permitted to lead to the exterior of the magnetic structure. In accordance with a feature of the invention the coil is airtight sealed relatively to the central magnetic core 1 and the exterior annular magnet 4 so as to provide a sealed air chamber between the magnetic supporting member and the membrane structure including the armature member 11.

When the device is mounted with the'membrane abutting against the deformed membrane supporting surface provided on the magnet supporting member, the air space between the membrane and the magnetic system is airtight sealed.

On the top surface of the magnet supporting washer there is secured a horse shoe formed insulating member 12 carrying a pair of terminal members 14a and 14b, either of which is connected with either of the two outlet wires extending through the slot 2a and the magnetic yoke 2. As well the horse shoe formed insulating member 12 as the terminal members 14a and 14b may be secured to the top surface of the magnet supporting washer by means of rivets or screws 24a and 24b which either may be of insulating material, or, in order to avoid short-circuiting, be conveniently insulated from the terminal members 14a and 14b by means of insulating washers surrounding the screwor rivet-heads and extending into the holes in the terminal members through which said screws or rivets extend.

As apparent from Figs. 2 through 6 the ends of the terminal members are bent at 14c and 14d vertical to the plane of the terminal members and are adapted to form contact surfaces to be engaged by the ends of the plug pins (not shown) of the plug h plugged in holes 15 of the exterior casing j.

This casing is assembled by two parts 16 and 17. The top part 16, in which the plug pin receiving holes 15 are provided, is preferably made of insulating material such as a convenient plastic material or any other thermoplastic mouldable material, which can be moulded or pressed.

This insulating part of the casing is provided with an annular flange having an exterior diameter substantially corresponding to the exterior diameter of the magnet supporting washer 5.

The bottom part of the exterior casing 17 may be of metal, preferably of a non-magnetizable material. This part is provided with an annular shoulder 18, outside which there is extending a thin annular portion 19. When mounting the parts together as shown in Figs. 3 and 4, the edge of the magnet supporting member will be clamped between the shoulder 18 and the flange 20 and by deforming the portion 19 the housing will be closed by means of the seam formed at the top edge of the portion 19. The portion 17 of the housing is provided with the outlet k in preferably alignment with the center of the membrane so as to transmit sound waves produced by the oscillations of the membrane member to the ear piece 1 fitting in the ear of the user.

In the plane view of Fig. 5, 13 refers to the holes through which the screws or rivets 24a, 24b are extending.

It will be appreciated that with the air-tight arrangement of the coil in the space between the center core 1 and the annular magnet 4 the space between the membrane structure and the magnet supporting structure is closed air-tight.

In order to enable a frequency response of the structure to a sensibility within a desired frequency range there may be provided in the magnet supporting member 5 a hole 21 in alignment with the hole 21a in the insulating horse shoe member 12.

With the dimensions hereinbefore referred to comprising an air gap between the membrane and the magnetic system of a magnitude of 17: 100 millimetre a totally closed air space will correspond to a maximum sensibility for frequencies between 1500 and 3000 cycles.

Now people needing hearing aid will have individual requirements for sensibilities in various frequency ranges. This is in one feature of the invention obtained by calibrating the hole 211. More specifically f. inst. a diameter of the hole 21 of 0.4 millimetre will change the maxi mum sensibility to an extended frequency range from 100 to 3000 cycles and further by way of example a 2.0 millimetre hole will change the sensibility to its maximum in a frequency range from 800 to 2000 cycles.

While the embodiment of Figs. 1 through 6 shows the device in accordance with the invention in form of a sound reproducer, the embodiment of Figs. 7 through 9 shows the device in form of a sound receiver or microphone, more specifically of the electrodynamic type.

Those parts of the structure according to Figs. 7 through 9, which have already been shown in Figs. 2 through 6 are referred to by the same reference numerals and will not be described in detail again.

Basically the embodiment of Figs. 7 through 9 differs from the embodiment of Figs. 2 through 6 by the following features.

The magnet supporting member is outside the uppressed portion 9, which forms the supporting surface for the metallic diaphragm 10, provided with a rim or shoulder forming a fiange 30 vertical to the plane of the magnet supporting member. Said flange 30 is extending in the same direction as the uppressed portion 9. The edge of the flange 30 is defining a plane parallel with the diaphragm member 10 and spaced therefrom axially in the direction of the center core 1. On this edge there is supported a further membrane 31 of a non-magnetic material such as aluminum.

The membrane 31 is at or near its center connected with the diaphragm member 10 in a convenient manner f. inst. by means of a pin 32, which may be soldered at one end to the armature member 11 and at the other end connected with the membrane 31. In the embodiment shown the end of the pin 32 remote from the armature member 11 is extending through a hole in the membrane 35 and is connected with this membrane by means of a suitable material such as a small piece of wax.

The unit formed in the manner here described is as indicated in Fig. 1 secured in a housing in a convenient manner f. inst. behind the openings of the housing as shown in Fig. 1 in which the embodiment of Figs. 7-9 is used as an electrodynamic microphone.

The membrane 31 is connected with the abutment edge in a convenient manner f. inst. by some sort of adhesive.

In order to provide the necessary resiliency of the membrane there is provided within the edge thereof an uppressed circular portion 33. The portion of the membrae 31 within this portion 33 will thus be free to move in response to the air pressure of the sound waves reaching the membrane, and the corresponding mechanical oscillations of the membrane 31 will by means of the pin 32 be transmitted to the armature member 11 and the diaphragm member 10 so as to thereby vary the air gap between the armature member and the pole surfaces of the magnetic system. Hereby an alternating current will be induced into the coil winding 3, and the output therefrom is through the coil outlet wires supplied to an amplifier built in the housing a.

This amplifier does not form part of the invention and is furthermore known in detail to those skilled in the art and will therefore not be described.

In order to obtain the necessary mechanical rigidness of the membrane 31 this may as shown in Figs. 7-9 be provided with a plurality of radially extending deformed ribs.

As previously mentioned an important feature of the present invention resides in using as a magnet supporting member a washer of a deformable material.

In production the washer is initially stamped out and thereafter provided with the depressed portion 5a serving as centering surface for the annular permanent magnet. In the same operation preferably the annular uppressed portion 9 is provided and given a height which is supposed approximately to support the armature member in the predetermined distance from the magnetic system after the mounting thereof.

Hereafter the magnetic system is secured on the surface of the washer in the position shown. The connection is preferably made by means of soldering. A convenient manner of performing the soldering is to apply soldering material to the one or the other of the surfaces to be connected and then place the washer with the annular magnetic member resting thereon between electric terminals, from which a strong electric current is sent through the soldering surfaces.

Hereafter the washer with the magnetic system secured thereto is arranged in a suitable gauge capable of measuring the air gap between the magnetic poles and the plane through the top of the uppressed annular portion 9. Though by the operation step in which said annular uppressed portion is provided it is as mentioned intended to give it an adequate height corresponding to the correct magnitude of the air gap this may not be achieved in the first operation, and if the gauge shows any variation with respect to the air gap the washer is subjected to a further depression adjusted in accordance with the measurement now effected, giving the uppressed portion the necessary height.

Hereafter the coil winding is mounted and the metallic diaphragm 10 with the armature member 11 is brought in position resting on the top of the annular uppressed portion 9.

The magnetic system is then or before bringing the diaphragm member in a position energized to such an extent that it attracts the membrane member wholly to contact the magnetic pole surface.

Hereafter an alternating current is applied to the coil winding. This current is of an amplitude beyond that for which the device is normally adapted to be operated.

The frequency of this alternating current may be around 300 cycles.

During the application of this alternating current the device is arranged in such a manner as to enable the acoustic output to be measured. F. inst. the opening k from the ear phone extends into a chamber in which is provided an air pressure meter of a convenient type such as f. inst. a condenser microphone.

The output from the condenser microphone is amplified in a convenient traditional amplifier and the output therefrom is applied to a convenient cathode ray oscillograph.

Such a measuring arrangement is generally known for the purpose of eifecting acoustic measurements, and those skilled in the art will with the brief information given here be able to design a corresponding measuring arrangement.

The output voltage from the sound pressure meter is shown as a curve on the screen of a cathode ray tube. As long as the magnetic system is energized to such an extent that the metallic diaphragm is capable of touch-.-

. 7 ing the pole surfaces, the voltage curve on the cathode ray tube screen shows a considerable distortion.

A de-energizer coil is then arranged adjacent the magnetic system and de-energizing thereof is effected preferably stepwise f. inst. by supplying de-energizing impulses.

After each deenergizing impulse the voltage curve on the cathode ray tube screen will clearly show a decrease of distortion. The de-energizing impulses are supplied subsequently until the voltage curve is a pure sine curve or in other way is responsive to the curve of the alternating current supplied to the coil winding of the device. At that stage there is no more distortion. The deenergizing'is interrupted and the adjustment is terminated.

As previously mentioned there may be provided holes 21 in the magnet supporting member in order to adjust the sensibility of the device in response to individual requirements.

When effecting such adjustment the hole or holes 21 are provided after the assembly of the device but before the de-energizing described 'hereinbefore.

It will be understood that the invention is not limited to the embodiment shown and described in that modifications in various respects can be allowed within the frames of the appended claims.

It will be noted that the construction provides a very simple and economical manufacture structure, which can be quickly assembled together when manufacturing with a minimum of labour and expense. I have provided a normal magnet unit and diaphragm which is particularly adapted for assembly with the parts described with a minimum of labour and expense.

Although not restricted thereto, in one commercial form of the present invention, the magnet 4 may be of a magnetic alloy known on the market under the name Alcomax manufactured by the British company Murex. The .annular magnet may have an internal diameter of 7 mm., an external diameter of 11 mm. and a height of 7 mm.

The center pole piece 1 may be of an alloy composed of approximately 40 percent nickel and 60 percent iron. Such an alloy is internationally known under the name of Radiometal. The center core 1 may be circular having a diameter of 3 mm. and a height of 4.5 mm.

The magnet supporting member 6 is of any sort of normal steel or iron. The blank is of a thickness of .5 mm. with an exterior diameter of between 21 and 22 mm.

The diameter of the annular portion 9 supporting the diaphragm is approximately 17.5 mm, and as previously mentioned this annular portion is uppressed so as to provide an air gap of approximately 17:100 mm.

The diaphragm 10 is made of steel in form of a blank having an exterior diameter of approximately 18.3 mm. and a thickness of .1 mm.

The armature member 11 secured to the diaphragm is of the same material, viz. Radiometal, as the center core member 1. The thickness is .35 mm. The diameter may vary from 7 mm. and up. In a convenient embodiment the diameter is slightly larger than the interior diameter of the annular magnet, preferably 8 mm., but it will be understood that the invention is not limited in this respect.

The magnetic yoke 2 is of the same material, viz. Radiometal as the center core 1.

The number of turns on the coil winding and the size of wire used will as known to those skilled in the art, of course, depend on the impedance of the coil. In one embodiment 350 turns of copper wire having a wire diameter of .09 mm. will correspond to an impedance of 50 ohm at 800 cycles.

It will be understood that many changes can be made in the specification given here without departing from the scope of the invention, and it Will therefore be understood that all matter contained in the above description or shown in the drawings shall be interpreted as illustrative only, and not in any limiting sense.

I claim:

1. In electromagnetic apparatus especially for hearing aid purposes in combination: a first magnetic member defining a substantially continuous wall having opposite end surfaces, a magnetic yoke extending across a substantial port of one end surface of said first magnetic member, a second magnetic member secured to said yoke and extending within the wall of said first magnetic member substantially concentric therewith and terminating substantially in alignment with the end surface of said first magnetic member which is remote from said yoke, a coil disposed Within the wall of said first magnetic member surrounding said second magnetic member, connection wires extending from said coil, a pole piece member of magnetizable material having an aperture of a configuration substantially corresponding to the interior configuration of that end surface of said first magnetic member which is substantially in alignment with said second magnetic member, said pole piece member being secured to said first magnetic member at an edge portion thereof surrounding said aperture, a positioning surface provided on said pole piece member, said positioning surface extending in a direction from said pole piece member opposite that of said first magnetic member and terminating in a plane which is spaced from said edge portion of said pole piece member surrounding said aperture, said plane being substantially parallel with said end surface of said first magnetic member, which is secured to said pole piece, a membrane supported on said positioning surface and an armature secured to said membrane substantially in concentric relationship with said first magnetic member.

2. In an electro-acoustic device in combination: a central magnet unit having an annular permanent magnet, a central permeable cylindrical iron core and a yoke member connecting said annular magnet and said core, a supporting plate ring having a substantially circular aperture corresponding to the interior diameter of said annular permanent magnet and supporting said central magnet unit extending from one .side of said ring, a metal diaphragm supported on an uppressed portion of said ring at the opposite side of said magnet unit in spaced relationship to said ring, a coil substantially filling the space between the annular magnet and the central core and connection wires from said coil extending outside said central magnet unit.

3. In an electro-acoustic device .in combination: a central magnet unit having an annular permanent magnet, a central permeable cylindrical iron core and a yoke member connecting said annular magnet and said core, a central mounting ring of a deformable material having a substantially circular aperture and an uppressed rim portion and supporting said central magnet unit extending from one side of said ring opposite said uppressed rim, a metal diaphragm supported on said uppressed rim portion of said ring at the opposite side of said magnet unit in spaced relationship to said ring, a coil substantially filling the space between the annular magnet and the central core and connection wires extending from said coil outside said central magnet unit.

4. In an electro-acoustic device in combination: a central mounting ring, a central magnet unit having an annular permanent magnet, a central permeable cylindrical iron core and a yoke member connecting said annular magnet and said core, said magnetic unit defining an annular exterior pole and an interior pole, said ring having a substantially circular aperture corresponding to the interior diameter of said annular permanent magnet, means supporting said central magnet unit on a surface of said ring in a position overlapping said annular exterior pole and with said magnet unit extending from one side of said ring, a metallic diaphragm supported on an uppressed portion of said ring at the opposite side of said magnet unit in spaced relationship to said ring, a coil substantially filling the space between the annular magnet and the central core and connection wires extending from said coil outside said central magnet unit.

7 5. In an electro-acoustic system in combination: a substantially circular fiat steel blank having a center aperture, an annular portion of said blank concentrically surrounding said center aperture being pressed slightly out of the plane of the body portion of said blank to define an annular rim, an electromagnetic system comprising an annular magnet, a center magnet, a magnetic yoke supporting said annular magnet and said center magnet and a coil winding supported inside said annular magnet and'surrouncling said center magnet, said electromagnetic system having said annular magnet secured to a surface of said blank with said annular magnet positioned around said aperture, and an armature structure comprising an armature member corresponding substantially to the aperture of said blank and a supporting membranecarrying said armature portion, said membrane being supported on said annular rim portion of said blank.

6. In an electro-acoustic system in combination: a substantially circular mounting steel ring having a center aperture, a first annular portion of said ring adjacent said center aperture being stamped slightly out of the plane of the body portion of said ring, a second annular portion of said ring concentrically surrounding said annular portion adjacent said aperture being stamped slightly more out of the plane of the body portion of said ring than said first annular portion, an electromagnetic system comprising an annular magnet, a center magnet a magnetic yoke connecting said annular magnet and said center magnet and a coil winding in the space between said magnets, means securing said annular magnet on a surface of said ring with said annular magnet soldered to the surface of said first annular portion of said ring adjacent said aperture, and an armature structure comprising an armature member corresponding substantially to the aperture of said ring and a supporting membrane carrying said armature portion, said membrane being supported on said second annular portion of said blank.

7. In an electro-acoustic system in combination: a substantially circular fiat steel blank having a center aperture, an annular portion of said blank concentrically surrounding said aperture being stamped slightly out of the plane of the body portion of said blank, said stampedout portion defining a positioning surface in spaced rela tionship to the body portion of said blank, an electromagnetic system comprising an annular magnet, a center magnet, a magnetic yoke connecting said annular magnet and center magnet and a coil winding in. the space between said magnets, means supporting said annular magnet and a surface of said blank with said annular magnet surrounding said aperture, a slot in said magnetic yoke, outlet wires from said coil winding extending through said slot in said yoke and an armature structure comprising an armature member corresponding substantially to the aperture of said blank and a supporting membrane carrying said armature portion, said membrane being supported on said stamped-out annular portion of said blank.

8. In an electro-acoustic system in combination: a substantially circular fiat blank of a magnetizable material having a center aperture, an annular portion of said blank concentrically surrounding and radially spaced from said aperture being stamped slightly out of the plane of said blank, an electromagnetic system comprising an annular magnet having an annular pole, a center magnet having a pole, a magnetic yoke connecting said magnets and a coil winding in the space between said magnets, means connecting said annular pole to a surface Of said blank with said annular magnet concentric with respect to said aperture, an armature structure comprising an armature member corresponding substantially to the aperture of said blank and a supporting membrane carrying said armature portion, said membrane being supported on said annular portion of said blank spaced from the poles of said magnetic system by an air gap of a predetermined size, and at least one aperture extending through said blank in communication with the space defined between said membrane, said blank and said magnetic system.

9. In an electro-acoustic device in combination: a central magnet unit having an annular permanent magnet, a central permeable cylindrical iron core and a yoke member connecting said annular magnet and said core, a supporting plate ring of magnetizable material having a substantially circular aperture corresponding to the interior diameter of said annular permanent magnet and an uppressed circular portion substantially concentric with said aperture and radially spaced therefrom, said ring being connected to and supporting said central magnet'unit in a position extending from one side of said ring concentric with said aperture in opposite direction of said uppressed portion, a metal diaphragm supported on said uppressed portion of said ring at the opposite side of said magnet unit in spaced relationship to said ring, an aperture in said magnet unit, a coil substantially fillingthe space between the annular magnet and the central .core, connection wires from said coil extending through said aperture in said central magnet unit, said aperture being substantially air-tight sealed so as to provide a closed structure comprising said magnet unit, said plate ring and said diaphragm and defining a cavity and at least one aperture extending through said plate ring into said cavity, said aperture being of a predetermined size to adjust the sensibility of the device to respond to a desired frequency range.

10. In an electro-acoustic device in combination: a central magnet unit having an annular permanent magnet, a central permeable cylindrical iron core and a yoke member connecting said annular magnet and said core, a plate ring supporting said central magnet unit in a position extending in one direction from said ring, an uppressed annular portion of said ring extending in opposite direction of said magnet unit, a metal diaphragm supported on said annular uppressed portion spaced by an air gap from said magnet unit, a coil substantially filling the space between the annular magnet and the central core and connection Wires from said coil extending outside said central magnet unit.

11. In an electro-acoustic device in combination: a central magnet unit having an annular permanent magnet, a central permeable cylindrical iron core and a yoke member connecting said annular magnet and said core, a supporting plate ring of magnetizible material having a substantially circular aperture correspondingto the interior diameter of said annular permanent magnet and supporting said central magnet unit on one side of said ring in concentric relationship with said circular aperture, a positioning surface defining a plane spaced from said magnet unit by means of an air gap, a metal diaphragm supported on said positioning surface of said ring, a coil substantially filling the space between the annular magnet and the central core and connection wires from said coil extending outside said central magnet unit.

12. In an electro-acoustic device in combination: a central magnet unit having a substantially annular permanent magnet, a central permeable cylindrical iron core and a yoke member connecting said annular magnet and said core, a plate ring supporting said central magnet unit extending in one direction thereof, a positioning surface on said plate ring, a metal diaphragm supported on said positioning surface of said ring in spaced relationship to said magnet unit, a coil substantially filling the space between the annular magnet and the central 11 core, connection wires from said coil outside said central magnet unit, a shoulder portion extending from the edge of said plate ring outside said diaphragm, said shoulder portion having an edge defining a plane substantially parallel with said diaphragm, a membrane supported on said shoulder edge and a mechanical connection between said membrane and said diaphragm.

13. In an electro-acoustic device in combination: a central magnet unit having a substantially permanent annular magnet, a central permeable cylindrical iron core and a yoke member connecting said annular magnet and said core, a plate ring of a deformable material supporting said central magnet unit extending in one direction thereof, an annular uppressed portion of said plate ring defining a positioning surface, a metal diaphragm supported on said positioning surface of said ring in spaced relationship to said magnet unit, a coil substantially filling the space between the annular magnet and the central core, connection wires from said coil outside said central magnet unit, a further uppressed portion adjacent the edge of said plate ring in form of a substantially circular flange having an edge defining a plane spaced from and substantially parallel with said diaphragm, a membrane supported on said edge, and a pin connecting said membrane and said diaphragm substantially at their centers.

14. In an electro-acoustic device in combination: a central magnet unit having a substantially annular permanent magnet, a central permeable cylindrical iron core and a yoke member connecting said annular magnet and said core, a plate ring of a deformable material supporting said central magnet unit extending in one direction thereof, an annular uppressed portion of said plate ring defining a positioning surface, a metal dia phragm supported on said positioning surface of said ring in spaced relationship to said magnet unit, a coil substantially filling the space between the annular magnet and the central core, connection Wires from said coil outside said central magnet unit, a further uppressed portion adjacent the edge of said plate ring in form of a substantially circular flange having an edge defining a plane spaced from and substantially parallel with said diaphragm, a membrane of non-magnetizable material supported on said edge, said membrane having adjacent said edge an annular resilient portion and within said resilient portion a reinforced center portion, and a pin connecting said membrane and said diaphragm substantially at their centers.

References Cited in the file of this patent UNITED STATES PATENTS 1,886,816 Kemsem Nov. 8, 1932 1,941,476 Jensen Jan. 2, 1934 1,941,477 Jensen Jan. 2, 1934 2,020,211 Quam Nov. 5, 1935 2,020,212 Quam Nov. 5, 1935 2,046,515 Jensen July 7, 1936 

